A proposed power output apparatus has an internal combustion engine, a planetary gear unit that includes a carrier linked with an output shaft of the internal combustion engine and a ring gear linked with a drive shaft, a generator that inputs and outputs power to and from a sun gear of the planetary gear unit, and a motor that inputs and outputs power to and from the drive shaft (see, for example, Japanese Patent Laid-Open Gazette No. H11-55810). In the case of heat generation in the generator or its driving circuit, this prior art apparatus decreases the output torque of the internal combustion engine and raises the revolution speed of the internal combustion engine, so as to reduce the load of the generator and prevent the heat generation in the generator while maintaining a power demand to be output from the internal combustion engine.
In the power output apparatus that independently drives the internal combustion engine and the drive shaft, in the case of the occurrence of some disturbance like overheat of the generator, the drive point of the internal combustion engine is changed to respond to the disturbance while keeping the power demand to be output from the internal combustion engine unchanged. Keeping the power demand to be output from the internal combustion engine unchanged maintains the driving force to be output to the drive shaft. When the motor that outputs power to the drive shaft is under a drive restriction due to heat generation in the motor or its driving circuit, however, the technique of keeping the power demand to be output from the internal combustion engine unchanged may make the output from the internal combustion engine significantly greater than the output to the drive shaft and may cause an accumulator like a secondary battery to be excessively charged.